In order for a thermally conductive adhesive composition to be useful in the manufacture of semiconductor devices, it should meet certain performance, reliability and manufacturing requirements dictated by the particular application. Such performance properties include strength of adhesion, coefficient of thermal expansion, flexibility, temperature stability, moisture resistance, electrical and thermal conductivity and the like. Thermal conductivity is of particular importance in the electronics industry. With the trend towards miniaturization coupled with higher operating frequencies, there are ever-increasing demands on engineers to remove heat from circuitry. The extraction of heat generated by components within a package is necessary to prevent those components from overheating. This is a larger problem for electronics containing high-power devices that can dissipate many watts of energy during normal operation.
In the prior art, die attachment adhesives generally comprised a silver flake or powder dispersed in a curable resin, such as an epoxy. However, such prior-art adhesives have thermal conductivities unsuitable for devices that dissipate large amounts of heat. Additionally, the prior art adhesives often have poor mechanical properties. Another disadvantage is that some prior art adhesives contain solvents to maintain low viscosity. During cure, such solvents have a propensity to form voids, requiring a long bake-out operation to drive off the solvent prior to cure. This adds time and cost to the overall cure process. Another shortcoming is that adhesives generally have unstable contact resistance after environmental aging. Heat and humidity also tends to reduce adhesion of conductive adhesives. Moisture absorption of conductive adhesives can lead to delamination failures during printed circuit assembly.
Few prior-art die attachment adhesives have the thermal conductivity suitable for use with high power devices. As a result, solder bonding is often the selected method. Solders have the advantage of having many times the thermal conductivity of most die attachment adhesives. Solders also have the advantage of the solder forming intimate metallurgical bonds with the devices being soldered. A metallurgical interface provides superior heat transfer compared to the typical adhesive interface.
However, solder bonding has a number of disadvantages. Solder preforms are usually employed to dispense solder between devices to be bonded, which are more expensive to apply during production than adhesive pastes. Another difficulty is that solder remelts if heated to an elevated temperature, yet elevated temperatures are required during electronic fabrication, e.g. during assembly of components to printed circuit boards. Such remelting of solder between components in a circuit can cause the parts to separate and subsequently fail.
Art related to adhesives is found in U.S. Pat. Nos. 6,613,123, 6,528,169, 6,238,599, 6,140,402, 6,132,646, 6,114,413, 6,017,634, 5,985,456, 5,985,043, 5,928,404, 5,830,389, 5,713,508, 5,488,082, 5,475,048, 5,376,403, 5,285,417, 5,136,365, 5,116,433, 5,062,896, and 5,043,102. Representative art directed to die attachment is found in U.S. Pat. Nos. 4,811,081, 4,906,596, 5,006,575, 5,250,600, 5,386,000, 5,399,907, 5,489,637, 5,973,052, 6,147,141, 6,242,513, and 6,351,340, and published PCT application WO 98/33645. The entire contents of all listed documents is hereby incorporated by reference.
There is clearly a need for a new composition that provides the best advantages of both solder and conductive adhesive. There is a need for a conductive adhesive that not only forms metallurgical bonds with the devices being bonded, but also provides significantly more thermal conductivity than is currently possible with silver powder-resin compositions while retaining high mechanical strength. Moreover, there is a need for a bonding material that hardens when used so that it does not remelt at elevated temperatures as well as possessing a high thermal conductivity, yet can be dispensed in paste form, without solvents, rather than preforms. Additionally, there is a need for a conductive adhesive that does not suffer delamination, reduced adhesion, or conductivity after aging, humidity exposure, etc.